This invention relates to shielded room construction for containment of fringe magnetic fields. More specifically, this invention relates to containment of fringe magnetic fields produced by a magnet which forms prt of a nuclear magnetic resonance (NMR) scanner.
The magnetic resonance phenomenon has been utilized in the past in high resolution NMR spectroscopy instruments by structural chemists to analyze the structure of chemical components. More recently, NMR has been developed as a medical diagnostic modality having application in imaging the anatomy, as well as in performing in vivo, non-invasive, spectroscopic analysis. As is now well known, the NMR resonance phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil is used to detect the NMR signals emanating from the patient volume lying within the field of the RF coil. The NMR signal is usually observed in the presence of linear magnetic field gradients used to encode spatial information into the signal. In the course of a complete NMR scan, a plurality of NMR signals are typically observed. The signals are used to derive NMR imaging or spectroscopic information about the object studied.
A typical whole-body NMR scanner used as a medical diagnostic device includes a magnet, usually of solenoidal design, having a cylindrical bore sufficiently large to accept a patient. The magnet is utilized to produce the polarizing magnetic field, which must be homogeneous typically to 1 part in a million for imaging applications and to in excess of 1 part in 10.sup.7 for spectroscopic studies. The field strength of the polarizing magnetic field can vary from 0.12 tesla (T) in electromagnets utilized for imaging applications to 1.5 tesla or more in superconductive magnets utilized for imaging as well as spectroscopic applications. It should be noted by way of comparison that the strength of the earth's magnetic field is approximately 0.7 gauss, whereas 1 tesla is equal to 10,000 gauss. Such strong magnetic fields, especially those in excess of 1T, are particularly useful in whole body NMR scanners. For specialized applications, suchas NMR spectroscopic studies, field strengths of 1T or greater are mandatory to detect useful NMR signals from such NMR-active nuclei as phosphorus (.sup.31 P) and carbon (.sup.13 C), for example.
Not unexpectedly, magnets capable of generating the field strengths referred to hereinabove, and having bores sufficiently large for accepting patients, generate fringe fields which can extend quite far from the magnet. Such fringe fields even at field strengths of 1 gauss can interfere with the normal operation of such devices commonly found in a hospital environment as computerized tomography (CT) scanners, nuclear tomographic cameras, and ultrasound systems. A fringe field strength of approximately 5 gauss is believed to have an adverse effect on cardiac pacemaker devices, neuro-stimulators, as well as other bio-stimulation devices. By way of illustration, the 5 gauss field can extend as far as 39 feet from the center of a magnet having a field strength of 1.5T and a 1 meter bore diameter. The necessity to contain the magnetic fringe fields, usually to 5 gauss, within the NMR scanner room is therefore apparent.
In the past, iron has been used to construct shielded rooms, housing the NMR scanner, for containment of magnetic field flux. However, conventionally designed shielded rooms have not made efficient use of the shielding material. Thus, for example, for containment of the 5 gauss field within a typical room for a 1.5T magnet system, the amount of iron needed can range from 50 to 90 tons. This can be a prohibitive amount of iron, due to economic and weight considerations, in situations where it is desirable to install an NMR scanner in an existing structure, as well as in new installations.
It is therefore an object of the invention to provide a technique for reducing the amounts of iron needed in the construction of shielded rooms.
It is another object of the invention to provide techniques for the construction of shielded rooms which effectively contain the MR field while making efficient use of shielding material.